Well drilling methods such as hydraulic fracturing method, rotary drilling method and riserless drilling method have now been widely employed for extracting underground resources.
The rotary drilling method consists of forming the well by drilling while refluxing the mud and forming a filter cake called mud wall on the wall surfaces of the well using a finishing fluid blended with a water loss-preventing agent. The cake maintains the chute walls stable, prevents the chute walls from collapsing and reduces friction to the fluid flowing through the well.
The hydraulic fracturing method consists of pressurizing the fluid filled in the well to form cracks (fractures) in the vicinities of the well to thereby improve permeability in the vicinities of the well (for easy flow of the fluid) in an attempt to increase the effective sectional area through which the resources such as oils and gases flow into the well and, therefore, in order to improve productivity of the well.
Here, as the water loss-preventing agent that is added to the finishing fluid, there are chiefly used calcium carbonate or various kinds of salts in a granular form. However, use of the water loss-preventing agent brings about such problems that it becomes necessary to conduct a treatment with acid to remove it, or the water loss-preventing agent stays clogged in the stratum from where the resources are to be extracted hindering the production.
Further, the fluid used in the hydraulic fracturing method is also called fracturing fluid. So far, a viscous fluid like jelly gasoline was used. However, as the shale gas or the like gas has now been extracted from the shale layer that exists in relatively shallow places and by taking the effects on the environment into consideration, it is becoming a common practice to use an aqueous dispersion solution obtained by dissolving or dispersing chiefly a polymer in water. A known example of the polymer is polylactic acid (see a patent document 1).
That is, the polylactic acid is a substance that exhibits hydrolysable capability and biodegradable capability, and, even if it remains under the ground, is decomposed by water or enzyme in the ground and does not adversely affect the environment. Further, the water that is used as a dispersant, too, can be considered to be far from affecting the environment as compared to gasoline or the like.
The well is filled with the aqueous solution in which the polylactic acid has been dispersed and is pressurized so that the polylactic acid permeates to the vicinities of the well. Here, the polylactic acid undergoes the hydrolysis and loses the form of the resin. Therefore, spaces (or cracks) form in the portions where the polylactic acid had been permeated accounting for an increase in the space of the well into which the resources can flow.
Further, the polylactic acid also works as a water loss-preventing agent and suppresses the water used as the dispersion medium from permeating into the ground too much. Therefore, the polylactic acid offers an advantage of minimizing a change in the environment in the stratum. Besides, no treatment with acid is necessary since it decomposes in the ground.
In addition, the lactic acid which is decomposed from the polylactic acid is an organic acid. As the polylactic acid decomposes, the lactic acid is released. The lactic acid corrodes the shale layer and accelerates the shale layer to become porous.
However, though the polylactic acid undergoes the hydrolysis relatively quickly at high temperatures, its rate of hydrolysis becomes small as the temperature decreases. If used for extracting, for example, the shale gas from under the ground where the temperature is low, therefore, the efficiency of extraction becomes poor and improvements are desired.
On the other hand, a proposal has been made to use a polyglycolic acid in place of the polylactic acid (see a patent document 2).
The polyglycolic acid, too, has been known to be used as a biodegradable resin. Besides, its hydrolysable capability is higher than that of the polylactic acid; i.e., the rate of hydrolysis at a temperature of, for example, about 80° C. is considerably larger than that of the polylactic acid and can be effectively used to substitute for the polylactic acid.
However, the polyglycolic acid is considerably expensive as compared to the polylactic acid. This is a fatal defect when it is used for the hydraulic fracturing method that uses the fracturing fluid in large quantities. Besides, the polyglycolic acid does not decompose to a sufficient degree under specific temperature conditions.